INTERACTION OF LANTHANUM, LUTETIUM, AND YTTERBIUM OXIDES AT 1600 ºС

Abstract

Phase equilibria in the ternary La₂O₃–Lu₂O₃–Yb₂O₃ system were studied by X-ray diffraction and electron microscopy for the first time at 1600 °С over the entire composition range. Solid solutions based on various crystalline modifications of the starting components and ordered perovskite-type phases were found to form in the system. The starting materials were La₂O₃, Lu₂O₃, and Yb₂O₃ (99.99%). The samples were prepared with a concentration step of 1–5 mol.%. Weighed oxides were dissolved in HNO₃ (1 : 1) and then the solutions were evaporated and decomposed at 800 ºC for 2 h. The samples were heat treated in three stages: at 1100 ºC (for 163 h), at 1500 °C (for 70 h), and at 1600 °C (for 10 h) in air. The samples were subjected to X-ray powder diffraction employing a DRON-3 diffractometer at room temperature (Cu-K_α radiation, Ni filter). The scan angle was 0.05–0.1° in the range 2θ =  15–90°. The isothermal section of the La₂O₃–Lu₂O₃–Yb₂O₃ phase diagram at 1600 °C shows three single-phase (A–La₂O₃, R, C–Lu₂O₃ (Yb₂O₃)) and two two-phase (C + R, A + R) regions. The system forms continuous areas of solid solutions based on the cubic modification of С-Lu₂О₃ (Yb₂O₃) and the ordered perovskite-type phase (R phase). The solubility limits were determined and composition dependences for lattice parameters the phases formed in the system were plotted. The boundary solubility of lutetium oxide in the R phase is 5 mol.% in the Lu₂O₃–(50 mol.% La₂O₃–50 mol.% Yb₂O₃) section. The homogeneity range of the C–Lu₂O₃ solid solutions extends from 93 to 100 mol.% Lu₂O₃ in the Lu₂O₃–(50 mol.% La₂O₃–50 mol.% Yb₂O₃) section. The solid solutions from through isovalent substitution, and the stability of ordered phases and solid solutions is determined by the shape factor of lanthanides.